1. Field of the Invention
The present invention relates to a photovoltaic device. More specifically, the present invention relates to a photovoltaic device such as a solar cell having, for example, an amorphous silicon derivative semiconductor photo-active layer.
2. Description of the Prior Art
FIG. 1 is a cross sectional view showing the fundamental construction of a conventional photovoltaic device in practical use. Such fundamental construction is shown, for example, in U.S. Pat. No. 4,281,208 assigned to the same assignee as the present invention. Transparent film electrodes 2a, 2b, 2c, --are formed on the surface of a substrate 1 having insulating and light transmissive properties, such as glass and heat-resistant plastic, etc corresponding to each photoelectric converting region 5a, 5b, 5c, --. The transparent film electrodes 2a, 2b, 2c, --are constituted by a transparent conductive oxide such as SnO.sub.2, ITO, and the like, and serve as first film electrodes. On each transparent film electrode 2a, 2b, 2c, --, a photo-active layer 3a, 3b, 3c, --is formed, which consists of a filmy amorphous semiconductor including amorphous silicon, amorphous silicon carbide, amorphous silicon germanium, and microcrystals thereof. On each photo-active layer 3a, 3b, 3c, --, a back film electrode 4a, 4b, 4c, --is deposited, which consists of an ohmic metal such as aluminum or aluminum alloy. Each back film electrode 4a, 4b, 4c, --extends toward an exposed portion 2b', 2c', --of the adjacent transparent film electrode 2b, 2c, --and is electrically connected thereto. Accordingly, a plurality of photoelectric converting regions 5a, 5b, 5c, --constituted by the transparent film electrodes 2a, 2b, 2c, --and the back film electrodes 4a, 4b, 4c, --sandwiching the photo-active layers 3a, 3b, 3c, --are electrically connected in series.
The photo-active layers 3a, 3b, 3c, --include therein, for example, a PIN junction parallel to the film surface or junctions of multiple construction or, so called tandem construction, etc. When light is irradiated through the substrate 1 and the transparent film electrodes 2a, 2b, 2c, --, electromotive force is generated in each photoactive layer 3a, 3b, 3c, --, which is electrically added and outputted.
For manufacturing a photovoltaic device having such a configuration, there has recently been proposed to use a laser beam as disclosed, for example, in U.S. Pat. No. 4,292,092. One technique disclosed in the U.S. patent is to form the adjacent spacing portions between each photoelectric converting region 5a, 5b, 5c, --by burning off the electrode film with irradiation by a laser beam. The laser beam process is very effective in that very precise processing can be obtained without using wet processing as in the previous photo-etching technique.
When using the laser beam, since the laser processing is substantially a thermal processing, care must be taken not to damage a layer which may be under the layer being processed. That is, thermal damage to the lower layer which need not needed be processed must be avoided, so that it is not removed in addition to the layer being processed. In order to satisfy this requirement, the prior art mentioned above proposes optimum setting of the laser etch conditions for each particular film, such as the laser output power, pulse frequency, etc.
The back film electrode 4a, 4b, 4c, --formed on each N-type layer of the respective photo-active layer 3a, 3b, 3, --is selected from well known metals such as aluminum, titanium, gold, silver, etc which make ohmic contact with the aforementioned N-type layer. Aluminum, however, is particularly advantageous for the following reasons. That is, when using aluminum, it is not only possible to reflect the light which penetrates through the N-type layer mentioned above, and the I-type (non-doped) layer contributing to the photoelectric conversion, but it also is low in cost. On the other hand, depending on the kind of laser to be used, since aluminum has a high reflection factor for ordinary laser beams, for example, more than approximately 90% for the YAG laser having a wave length of 1.06 .mu.m, and a good heat conductivity, patterning by the laser beam without causing thermal damage to the exposed portions 2a', 2b', 2c', --of the transparent film electrodes 2a, 2b, 2c, --is difficult. Also in the U.S. patent cited above, in the embodiment using aluminum as the back film electrode material, a method of direct pattern forming without laser processing is employed by vapor depositing of aluminum from a diagonal direction onto the photoactive layer 3a, 3b, 3c, --and the transparent film elctrodes 2a, 2b, 2c, --where the laser patterning has been previously made.
Furthermore, a difficulty which tends to occur in thermal processing by irradiation of a laser beam is that, an unremoved "hanging" edge 7 of the back electrode film is the adjacent spacing portion 6 remains as shown in FIG. 2, because of the good heat conductivity of the back film electrode 4b made of the ohmic metal. When this "hanging" of the edge 7 takes place, since the exposed portion 2b' of the transparent film electrode 2b of the associated photoelectric converting region 5b is present thereunder, the photoelectric converting region 5b will be electrically short circuited by the contact therebetween.
Further care must be taken so that, as shown in FIG. 3, in the construction wherein the back film electrodes 4a, 4b, 4c, --are separated on the photo-active layers 3a, 3b, 3c, --the back electrodes 4a, 4b, 4c which shoud be physically separated via a portion 3' of the photo-active layer are not connected electrically due to the portion 3' of the photoactive layer being hit directly by the laser beam, causing annealing and lowering the resistance thereof.